1. Field of the Invention
The present invention relates to a magnetoresistive device adapted to read the magnetic field intensity of magnetic recording media or the like as signals, and a thin-film magnetic head comprising that magnetoresistive device as well as a head gimbal assembly and a magnetic disk system, one each including that thin-film magnetic head.
2. Explanation of the Prior Art
In recent years, with an increase in the longitudinal recording density of magnetic disk systems, there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magnetoresistive device (hereinafter often called the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together.
The MR device, for instance, includes an AMR device making use of the anisotropic magnetoresistive effect, a GMR device harnessing the giant magnetoresistive effect, and a TMR device tapping the tunnel-type magnetoresistive effect.
The reproducing head must have some characteristics in general, and high sensitivity and high output in particular. For the reproducing head capable of meeting such demands, there has already been a GMR head mass produced that makes use of a spin valve type of GRM device.
Such a spin valve type GMR device generally comprises, as part of the device, a nonmagnetic layer, a first magnetic layer (the so-called free layer) formed on one surface of the nonmagnetic layer, a second magnetic layer (fixed magnetization layer) formed on another surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) formed in contact with the fixed magnetization layer that faces away from the nonmagnetic layer. The free layer operates such that the direction of magnetization changes in response to a signal magnetic filed coming from outside, and the fixed magnetization layer has the direction of magnetization fixed by an exchange coupling magnetic field from the pinning layer (antiferromagnetic layer). With such device structure, MR changes are achievable via a difference in the relative angle of spins in two such ferromagnetic layers.
As head size diminishes, there is a mounting demand for an arrangement capable of increasing the MR ratio, and the so-called current narrowing type head has been proposed as one of the prior arts capable of obtaining large MR ratios while resistance is kept low (see JP(A)2005-243154 (JP Patent No. 3993175)). However, that head structure is not compatible with a demand for drastic decreases in the head area: much difficulty is encountered in achieving sufficient size reductions, with high precision, of a Cu-pillar located in a spacer and used for narrowing currents. For this reason, it is difficult to hold back characteristics variations for each device, and much is still desired for the reliability of the device itself, offering an obstacle to practical use.
JP(A)2003-8102 that is another prior art discloses a CPP-GMR device that comprises a fixed magnetization layer including a magnetic film having the direction of magnetization substantially fixed in one direction, a free magnetization layer including a magnetic film having the direction of magnetization changing in response to an external magnetic field, a nonmagnetic metal intermediate layer interposed between the fixed magnetization layer and the free magnetization layer, and a resistance adjustment layer interposed between the fixed magnetization layer and the free magnetization layer and formed of a material having up to 1022/cm3 of conduction carriers. According to this proposal, the problem with characteristics variations for each device is eased off because of no use of any Cu-pillar for the formation of the narrowing structure. In addition, adjustment of device resistance enables a proper area resistivity to be achieved. However, the effect on improvements in the MR ratio is limited because the MR ratio depends largely on the nature of the nonmagnetic metal intermediate layer.
To solve such a problem with JP(A)2003-8102, the inventors have already filed JP(A)2008-177272 to propose a CPP-GMR device wherein 1 to 8 at % of a given metal are contained in a semiconductor oxide layer forming a part of the spacer layer.
According to this proposal, the addition of the given metal in an amount of at least 1 at % has the features of maintaining ohomic conduction and reducing shot noises, and the proper area resistivity is achieved, too. However, there is no end to the demand for MR ratio improvements: much more improvements are still now in great demand.
The situations being like this, the present invention has been made for the purpose of providing a novel magnetoresistive device that is just only capable of overcoming the aforesaid problems with the prior art but also can make the MR ratio high while holding back an increase in the area resistivity, and a thin-film magnetic head comprising that magnetoresistive device as well as a head gimbal assembly and a magnetic disk system, one each comprising that thin-film magnetic head.